Field of the Invention
The present invention generally relates to photo-stable highly luminescent chromophores which are useful in various applications, including in wavelength conversion films. Wavelength conversion films have the potential to significantly enhance the solar harvesting efficiency of photovoltaic or solar cell devices. In particular, the chromophores described herein are useful for wavelength conversion of visible and near infrared radiation.
Description of the Related Art
In recent years, with the need for new optical light collection systems, fluorescence-based solar collectors, fluorescence-activated displays, and single-molecule spectroscopy, various approaches for preparing luminescent chromophores have been explored. However, many technical issues have yet to be overcome.
In the form of monomers, oligomers or polymers, 2H-benzo[d][1,2,3]triazole derivatives have recently become of high interest due to their usefulness in designing electrochromic and optoelectronic devices. Typical derivatives of 2H-benzo[d][1,2,3]triazole from the literature are composed of electron-donating substituents at the N-2, C-4 and C-7 positions, as shown below in Structure A, where typical electron-donors described in the literature comprise alkyl 1-hydroxyphenyl and 1-alkoxyphenyl groups, and typical electron donor groups at N-2 described in the literature comprise alkyl, 2-hydroxyphenyl, and 2-alkoxyphenyl. These derivatives exhibit strong absorption and fluorescence bands in UV and/or short visible wavelength region.

Chromophores based on 2-alkylbenzotriazole scaffold have also become of significant interest in recent years. The majority of these types of compounds comprise 2-alkyl-4,7-dithienylbenzotriazole units, similar to Structure B, shown below, with various substituents, usually in the form of polymers and copolymers.

Chromophores based on benzothiadiazole, shown by Structure C, are the most common, especially with thienyl groups as electron donors, which are convenient building blocks for fluorescent polymers.
While much of the work on organic luminescent dyes has focused on the two ring core structure described above, there has been very little work reported on luminescent dyes with three ring cores. Some initial studies of compounds with the following three ring core structures have been reported:

where X is N, S, or Se, and D1 and D2 are hydrogen, furan, or thiophen, and are described in the following publications: Japanese patent applications JP19810054380 and JP19810107409, Organic Letters 2011, 13(17), 4612, Journal of Materials Chemistry 2012, 22, 4687, Organic Letters 2011, 13(9), 2338, Spectrochimica Acta Part A 2007, 66, 849, Spectrochimica Part A 2004, 60, 2005, Journal of Chemical Information and Modeling 2011, 51, 2904, Chem. Commun. 2012, 48, 1236, Organic Letters 2012, 14(2), 532, Journal of Organic Chemistry 1986, 51, 979, Tetrahedron Letters 1984, 25(20), 2073, Polymer 2012, 53, 1465, Journal of the American Chemical Society 2012, 134, 2599.
One of the useful properties of fluorescence (or photo-luminescent) dyes is that they have the ability to absorb light photons of a particular wavelength, and re-emit the photons at a different wavelength. This phenomenon also makes them useful in the photovoltaic industry. The utilization of solar energy offers a promising alternative energy source to the traditional fossil fuels, and therefore, the development of devices that can convert solar energy into electricity, such as photovoltaic devices (also known as solar cells), has drawn significant attention in recent years. Several different types of mature photovoltaic devices have been developed, including a Silicon based device, a III-V and II-VI PN junction device, a Copper-Indium-Gallium-Selenium (CIGS) thin film device, an organic sensitizer device, an organic thin film device, and a Cadmium Sulfide/Cadmium Telluride (CdS/CdTe) thin film device, to name a few. However, the photoelectric conversion efficiency of many of these devices still has room for improvement and development of techniques to improve this efficiency has been an ongoing challenge for many researchers.
One technique developed to improve the efficiency of photovoltaic devices is to utilize a wavelength conversion film. Many of the photovoltaic devices are unable to effectively utilize the entire spectrum of light as the materials on the device absorb certain wavelengths of light (typically the shorter UV wavelengths) instead of allowing the light to pass through to the photoconductive material layer where it is converted into electricity. Application of a wavelength conversion film absorbs the shorter wavelength photons and re-emits them at more favorable longer wavelengths, which can then be absorbed by the photoconductive layer in the device, and converted into electricity.
While there have been numerous disclosures of wavelength conversion inorganic mediums used in photovoltaic devices and solar cells, there has been very little work reported on the use of photo-luminescent organic mediums for efficiency improvements in photovoltaic devices. The use of luminescent down-shifting materials to improve the efficiency of photovoltaic devices and solar cells has been disclosed in several publications, including U.S. Pat. No. 7,791,157, and U.S. Patent Application Publication Nos. 2009/0151785, 2010/0294339, and 2010/0012183. All of these publications include example embodiments of luminescent down-shifting mediums applied to a photovoltaic device or solar cell in which the medium is composed of an inorganic material. The use of an organic medium, as opposed to an inorganic medium, is attractive in that organic materials are typically cheaper and easier to use, making them a better economical choice. However, most of the currently available organic luminescent dyes are typically not photo-stable for long periods of time, and therefore unusable in photovoltaic applications which require consistent performance for 20+ years.